Electrically tunable, liquid-crystal, optical filters have been proposed, for example, by Patel et al. in "An electrically tunable optical filter for infra-red wavelength using liquids crystals in a Fabry-Perot etalon," Applied Physics Letters, volume 57, 1990, pp. 1718-1720 and by Patel in U.S. patent application, Ser. No. 07/677,769, filed Mar. 29, 1991. Although different types have been proposed, the high-performance types share the structure illustrated in FIG. 1 for a liquid-crystal etalon filter 10. Two dielectric interference mirrors 12 and 14 are formed on transparent substrates 16 and 18 as two separate assemblies. Semi-transparent electrodes 22 and 24 are deposited on the mirrors 12 and 14. The two assemblies are then fixed together with a small predetermined gap between them, and a liquid crystal 26 is filled into the gap. The size of the gap is chosen such that the corresponding optical length between the mirrors 12 and 14 (taking into account the relevant refractive index of the liquid crystal 26) nearly equals the wavelength of the light being filtered or a multiple thereof. That is, the mirrors 12 and 14 and intervening liquid crystal 26 form a Fabry-Perot cavity and thus an etalon filter for transmitted light. A voltage generator 28 electrically tunes the liquid-crystal by imposing a variable voltage, determined by a tuning signal TUNE, across the electrodes 22 and 24 and thus imposing an electric field across the liquid crystal 26. At least one of the refractive indices of the liquid crystal 26 is changed by the electric field. Thereby, the optical length of the resonant cavity is changed, and the filter 10 will pass an optical band of the input light 20 into an output light 30 in correspondence to the voltage imposed across it. This description has neglected alignment layers adjacent to the liquid crystal and polarizing components which vary among the various liquid-crystal filters, but preferred examples may be found in the Patel references.
A liquid-crystal filter of this type is not only easy to fabricate and to operate, it also offers a very narrow bandwidth of the order of 1 nm because of the high reflectivity (greater than 98%) and the low loss provided by the dielectric interference mirrors. However, this narrow bandwidth raises difficulties. The refractive indices of the liquid crystal depend not only on electric field but also upon the temperature of the liquid crystal. Some experiments, to be described later, have determined that a temperature variation of .+-.0.5.degree. C. can shift the pass band by as much as half the width of the pass band. Although temperature can be controlled to these small variations, such controlling equipment is expensive and limits the usefulness of liquid-crystal etalon filters.